Field
The disclosure relates to a fuel cell system and its start-up method. More specifically, the present disclosure relates to improvements in configurations of a fuel cell system that may start at a below-freezing temperature and in its start-up method.
Description of Related Art
In general, it takes a certain length of time for a fuel cell system to reach a state where a predetermined electric power can be supplied (i.e., a start-up completion state called a “Ready On” state). Normally, when the fuel cell is started, a warm-up operation for raising the temperature of the fuel cell to a temperature suitable for power generation is performed. This warm-up operation is often performed by self-heating of the fuel cell without using an external heater or the like that raises the temperature of the fuel cell, in consideration of the efficiency of the fuel cell system.
Conventionally, various techniques for rapidly starting fuel cells using self-heating as described above have been proposed. For example, in order to rapidly raise the temperature of a fuel cell during start-up in a low-temperature condition, the flow rate of a coolant to be circulated in the fuel cell is reduced or the circulation of the coolant is stopped. Alternatively, in contrast to such techniques, the flow rate of the circulating coolant may be increased in the middle of start-up operation of the fuel cell (see, for example, JP2013-218923 A).
However, since in many cases a fuel cell immediately after start-up at a below-freezing temperature has been left uncontrolled in a below-freezing condition until the start-up operation is started, water that remains in a part of a MEGA (Membrane Electrode and Gas Diffusion Layer Assembly) is frozen and even if the various techniques for rapidly starting the fuel cell are employed, it still takes a remarkable length of time to start the fuel cell. Under such circumstances, the larger the amount of frozen water is, the longer it takes to unfreeze it. In addition, the lower the temperature of the fuel cell at the time of start-up is, the longer it takes to raise the temperature. Thus, a user may get frustrated or feel uncomfortable since he/she has to wait before the fuel cell reaches the Ready-on state where a predetermined output (a Ready-on output) can be provided, after start-up at a below-freezing temperature.
Furthermore, because of the fact that, if a power-generating area has been reduced due to freezing, the power generation performance tends to be degraded, even if a fuel cell hybrid vehicle is turned into a WOT (Wide-Open Throttle: a state in which a throttle (throttle valve) is fully opened) state after the Ready-on state is reached, an output and increase in the output may be less than those during a start-up operation at ordinary temperatures. Thus, even if an accelerator pedal is pressed, a desired output desired or expected by a user may not be obtained and this may make the user feel strange. In such situation, if information regarding such phenomenon has been previously provided to the user, the user may not mind very much even if there is a deviation between the desired output (the output the user desires or expects when he/she presses the accelerator pedal) and the actual output. However, in fact, the user cannot help but feel strange as there is a significant deviation therebetween.